The present invention relates to the manufacture of glassware articles, and in particular to manufacture of such articles of improved strength.
It has been well known for many years that the strength of glassware articles such as glass containers is limited by surface flaws which act as stress raisers and points of failure if the glass surface is put into tension. Many proposals have been made for methods of increasing the strength of glass containers by treatment of the containers and these proposals have included many in which attempts have been made to generate a surface layer on the glass container which surface layer is in compression.
One example of such a process is the dipping of the glass container into an ion exchange bath consisting of a mixture of molten salts in order to exchange sodium ions in the surface layer of glass for the larger potassium ions.
Another proposal has involved spraying of the glass article with a salt solution containing potassium and then heat treating the sprayed glass container in order to effect an exchange of sodium ions in the glass for the larger potassium ions.
In both these cases the introduction of the larger potassium ions in place of sodium ions in the surface glass places the surface of the glass container in compression. However, these processes suffer from the disadvantage that a long time is required to effect the ion exchange because the process is carried out at temperatures below the glass transition temperature. Increasing the temperature of treatment reduces the time significantly but there is an increased risk that the articles will distort in an unacceptable manner. This temperature limitation also prevents the use of other strengthening processes which otherwise could be considered.